Fitness Consequences of Reciprocally Asymmetric Hybridization between Simultaneous Hermaphrodites

Abstract

Depending on fitness consequences, hybridization may rescue inbred populations; generate premating barriers, reproductive interference, or hybrid species; or extinguish a species. However, the fitness of hybrids is unpredictable without direct quantification of their performance in fitness components across multiple generations. The land snails Bradybaena pellucida and B. similaris, which are indigenous and non-indigenous in Japan, respectively, copulate with each other simultaneously and reciprocally. However, only B. pellucida produces hybrids, because it ends mating by removing the penis before transferring a spermatophore, while B. similaris inseminates B. pellucida. To evaluate the strength of an intrinsic postzygotic barrier against the hybrids produced by B. pellucida, we conducted breeding experiments in the laboratory and measured six life-history traits: (1) growth rate, (2) body weight at maturity, (3) number of days to first oviposition after being permitted to mate, (4) clutch size, (5) fecundity, and (6) hatchability. We also calculated the relative intrinsic fitness based on five of these trait values (excluding clutch size). F1 hybrids exhibited heterosis in growth rate, body weight at maturity and relative intrinsic fitness. F2 hybrids also showed heterosis in body weight at maturity. Nevertheless, the F2 hybrids produced significantly fewer progeny than the mid-point value of the parental species. Thus, the F2 hybrids exhibited weak outbreeding depression in reproduction, offsetting their vigor in body size. These results indicate that only a weak postzygotic barrier, contrasting with strong F1 heterosis, has evolved during genetic divergence of the two sibling species in allopatry.

Divergence before and after the isolation of islands: Phylogeography of the Bradybaena land snails on the Ryukyu Islands of Japan

“…When interspecific copulation occurs, B. pellucida removes its penis without transferring a spermatophore, while B. similaris sires hybrids by inseminating B. pellucida (i.e. reproductive interference; Wiwegweaw, Seki, Mori, & Asami, ; Wiwegweaw, Seki, Utsuno, & Asami, ). Therefore, the difference in the micro‐sculpture patterns on the penial wall of these species might be associated with a function in physical mate recognition during copulation among Bradybaena , including the species in the Ryukyu Islands.…”

Partial precopulatory isolation between two geographically distant populations of the land snail Arianta arbustorum (L.)

“…Thus, reproductive compatibility was still maintained as already reported in Baur & Baur (1992a). The study design chosen ( paternity analysis) did not allow us to examine the survivorship and fitness of the emerged offspring, which could be influenced by outbreeding depression (Wiwegweaw et al, 2009b). However, we recorded a reduced hatching success in eggs of focal snails which copulated with one heterotypic and one homotypic partner compared to eggs of focal snails which mated twice with homotypic partners.…”

“…However, in A. arbustorum outcrossing is preferred to self-fertilization even under outbreeding circumstances, as observed in this study. This result indicates that potential negative fitness consequences due to outbreeding depression are less costly than inbreeding depression as a result of self-fertilization (Frankham, 1995;Wiwegweaw et al, 2009b).…”

Mitochondrial and nuclear DNA analysis revealed a cryptic species and genetic introgression in Littorina sitkana (Mollusca, Gastropoda)

“…The exceptional associations between HSC70 and mtDNA haplotypes (7% of genotyped samples from Hokkaido had CBb ? HSCa, see Table 3) can probably be attributed to genetic introgression of mitochondrial DNA caused by occasional hybridization after separation between L. sitkana and CladeB, as observed in many other species (e.g., Lamb and Avise 1986;Mikhailova et al 2009;Reid et al 2006;Shimizu and Ueshima 2000;Yamazaki et al 2008;Wiwegweaw et al 2009). Nevertheless, such hybridization events found in the present study are unlikely to have occurred recently because no HSCa and HSCb heterozygotes were observed in our investigation.…”